>   The reason for that artifact is a rather simple one, but I can't think
> of a good solution in the general case. In this specific case there's
> probably a working solution.
>
>   The problem is that at that specific point (the center of the circle)
> the radiosity samples are taken from a point which is almost exactly at
> the corner of the room, ie. at the junction of the two walls.
>   That is, that point is practically in the same place as the wall on
> the right.
>
>   Now, think how the radiosity sampling works: POV-Ray sends semi-random
> rays from that point to all directions (well, in all directions of the
> hemisphere pointing out of the wall surface).
>   Since the sampling point is located just at the junction of the two
> walls, some of these sample rays miss the right wall and are actually
> going outside the room. Since it's dark outside, it affects the lighting
> of that point.


Thank you for this detailed description!


>   I really can't think how this problem could be fixed in the POV-Ray
> source code.
>   However, you could try translating your camera by a minuscule amount
> (like 0.001) to some direction (but not vertically, because the walls
> are vertical at the problematic place), and it might correct the problem.
> It's enough for the sampling point to be taken just slightly from a different
> place.
>   If you are out of luck, you might get a similar artifact somewhere else.
> If this is the case, try translating the camera by a different amount.


Mhh. That's not what I hoped for. Rendering such an image takes more than an
hour, and (repeatedly) moving the camera until no artifact appears can be
frustrating.


>   Another possible solution (which is not a generic one but which probably
> works here) is to make the right wall shadowless and put a copy of it
> behind it (very close to it, but not too close). This way the sample
> rays going outside the room will hit this second wall (which will be
> illuminated by the light source because the original wall is now
> shadowless).


Well, if the geometry of the "room" is simple, that could be a solution.
When I get home, I'll try if it has any sideeffects...


>   By the way, can I use this image (well, a section of it) to make a new
> entry in the http://tag.povray.org/povQandT/ site explaining this problem?


Sure, no problem. If you need the original (as png without the arrow), mail
me.

Manuel

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